Small two band radiators for frequency bands around 900 MHz and 1800 MHz are available commercially although they are not sufficiently broadbanded to reach to frequencies of 2200 MHz. Further, very high broadband antennas are available, for instance the logarithmic periodic antennas, although these are too large and expensive for simpler applications.
Described in EP 0 613 209 A1, with the title xe2x80x9cA two frequency impedance matching circuitxe2x80x9d is a matching circuit for a simple whip antenna that enables roughly 50 Ohms matching at two frequencies to be achieved. In the preferred embodiment, these frequencies lie between 810 and 960 MHz. This implies that the antenna is broadbanded within this frequency band; see FIG. 7 of the patent specification.
The present invention has a different aim, as matching is strived for in two frequency bands that are widely separate from each other, of which at least one band is very wide.
The object of the invention is to provide a radio antenna that includes a matching circuit which functions on at least two different frequency bands, of which at least one is broad. Other objects are that the antenna shall be relatively small in relation to alternative solutions, and that it shall be relatively simple and economic to mass-produce. For instance, the matching circuit can be mounted on printed circuit boards. In some cases, the radiator may also be mounted on the same printed circuit board.
The invention has evolved as a result of the need to transmit and receive radio waves with a single antenna, within all of the following communications frequency band:
The invention will be described with reference to two preferred embodiments for these frequency bands. However, the invention can be also applied for other frequency ranges and other applications, and hence the principle of the invention will be described in more generality in the accompanying claims.
The frequency ranges with which the preferred embodiments are concerned will be designated in accordance with the following:
890-960 MHz will be referred to as the xe2x80x9cfirst frequency bandxe2x80x9d
1710-2200 MHz will be referred to as the xe2x80x9csecond frequency bandxe2x80x9d
In this case, the three higher frequency bands have been combined into a broader band.
A complete antenna consists of radiator (5, 20) and matching circuit (8). The matching circuit always includes a transmission circuit (10, 21) and, when required, a Balun transformer (9). It is assumed that the radiator has low radiation resistance within a first frequency band and a high radiation resistance within a second frequency band.
The purpose of the transmission circuit (10, 21) is to transfer the electromagnetic wave from the antenna connection point, Port Axe2x80x94A, to the other end of the transmission circuit, Port Fxe2x80x94F, so that its impedance values within both frequency bands will lie in the proximity of a common resistance value that corresponds to the impedance of the feeder, the Balun transformer, or the radio apparatus, illustrated at point O in the Smith chart of FIGS. 4 to 8 inclusive. When the antenna is balanced (e.g. dipole) and the supply line is unbalanced (e.g. a coaxial cable), the matching circuit (8) will also include a Balun transformer (9) whose Port Gxe2x80x94G is matched to the unbalanced feeder.
The function of the transmission circuit is illustrated in the Smith chart in FIGS. 4-9, where it is shown how the impedance curve of the radiator is changed incrementally, so that the curve segment which lies in the frequency bands concerned is moved to the proximity of the centre point O in the Smith chart of FIG. 9.
The impedance of the radiator (5) in the Smith chart (FIG. 4) shows that the curve intersects the true axis at a first point (P) within the first frequency band, and at a second point (Q) within the second frequency band. The radiator (5) is thus resonant at these frequencies. The curve is moved down in the capacitive region of the Smith chart shown in FIG. 5, with the aid of parallel capacitor (11). The inductance (13) moves the curve to the inductive region and draws the curve together to form a small loop in accordance with FIG. 6. The curve is moved closer to the centre point (O) of the diagram in FIG. 7, with the aid of series capacitor (12), and its balance is improved in relation to the horizontal axis (X) of the diagram at the same time. The curve is then shifted through a phase angle of about 130xc2x0 with the aid of a phase shift line (14), the result being shown in FIG. 8. We see here that the markers in the first band lie in the proximity of the horizontal axis (X), whereas the markers for the second band lie on a coherent loop in the inductive part of the Smith chart. This last-mentioned loop is moved with the aid of the parallel capacitance (15), so that it will lie around the centre point (O) in the Smith chart, see FIG. 9. The region for the first band is therewith influenced only to a small degree, as the parallel capacitance (15) influences the positions of the points in the Smith chart to a smaller degree at these low frequencies. Thus, as seen from the first port (Axe2x80x94A) to the second port (Fxe2x80x94F), the transmission circuit (10) is comprised of parallel capacitor (11), series inductance (13), series capacitor (12), phase shifting line (14) and parallel capacitor (15), in that order.
The Balun transformer (9) will be described below in conjunction with the first preferred embodiment.
Small two band radiators for frequency bands around 900 MHz and 1800 MHz are available commercially although they are not sufficiently broadbanded to reach to frequencies of 2200 MHz. Further, very high broadband antennas are available, for instance the logarithmic periodic antennas, although these are too large and expensive for simpler applications.
Described in EP 0 613 209 A1, with the title xe2x80x9cA two frequency impedance matching circuitxe2x80x9d is a matching circuit for a simple whip antenna that enables roughly 50 Ohms matching at two frequencies to be achieved. In the preferred embodiment, these frequencies lie between 810 and 960 MHz. This implies that the antenna is broadbanded within this frequency band; see FIG. 7 of the patent specification. The present invention has a different aim, as matching is strived for in two frequency bands that are widely separate from each other, of which at least one band is very wide. 3